Microstrip array antennas transmitting or receiving circularly polarized electromagnetic waves in the microwave and millimeter wave range are extensively used in communications systems such as mobile-satellite communications, direct-broadcasting-satellite systems, navigation and radar systems. They are particularly useful where the antenna resides on a moving platform, e.g. an automobile, truck or a spacecraft, which must be in constant communication with its counterpart on another platform which may be either stationary or moving.
Circular polarization is usually achieved by combining two orthogonal linearly polarized waves which are equal in amplitude and are radiating in phase quadrature relation. The tip of the radiated electric field vector rotates in a circle in the plane transverse to the direction of propagation and is right circular polarized when rotating clockwise and left circular polarized when rotating counterclockwise looking in the direction of propagation. Performance requirements of the communication system dictate the design for the particular microstrip antenna characteristics and often the conventional circularly polarized microstrip antenna is comprised of an array of microstrip radiating elements when the required gain is higher than that of a single radiating element.
The conventional method of obtaining a circularly polarized array is to arrange circularly polarized microstrip patches with appropriate feeding. Various types of circularly polarized patches are used as array elements and include those which can support two orthogonal (in space) modes of excitation, more common ones being circular or square in shape. These two orthogonal resonant modes are excited with equal amplitude and in phase quadrature (differential phase shift of 90.degree.) with dual feed to produce the appropriate sense of circularly polarized radiation. However, by means of an appropriate structural perturbation to the circular polarizable radiating patches, it is possible to excite circular polarization of the appropriate sense by means of a single feed point excitation. While the required length of feed lines is reduced, the single feed excitation is fundamentally inferior to dual feed excitation in terms of antenna performance such as measured by axial ratio bandwidth. This is so because at a frequency slightly off resonance, the amplitude and phase differential between the two orthogonal linearly polarized fields will always be much larger than when using dual feed excitation because of the steep slope of the impedance resonance curve at frequencies off-resonance.
Microstrip radiators may be excited by direct feeding or indirect feeding. There are essentially two ways of direct feeding. One is to use coplanar microstrip line feed and the other is to use perpendicular coaxial feed with a pin exciting the microstrip from the bottom. There are also two ways of indirect feeding the microstrip radiators. One is by means of electromagnetic or capacitive coupling through one or more dielectric layers and the other through an aperture in a conducting surface below the microstrip and separated by one or more layers of dielectrics from the feed. The aperture, in turn, could be fed by a microstrip feed line one or more dielectric layers below the aperture.
The working of a practical circularly polarized microstrip array antenna is characterized by several important performance parameters which include the radiation gain pattern, impedance bandwidth, axial ratio bandwidth, antenna efficiency and side lobe level. When electronic scanning by a full phased array or subarray is involved, maximum available scan angle and the variations of gain, beamwidth, axial ratio, side lobe level and antenna input impedance with scanning are also important. Antenna efficiency that tells how much of the antenna input power is converted into useful output power for communication is a very important performance measure. Signal power losses in the feed structure decreases the antenna efficiency. Lower efficiency for a transmitting array antenna means lesser signal power is radiated whereas lower efficiency for a receiving array antenna means more noise is introduced in the captured signal adversely affecting the signal detection capability of the communication system. Axial ratio bandwidth is a measure of the operational frequency range over which the desired sense of circular polarization remains useful. Impedance bandwidth of the antenna array is the operational frequency range over which the antenna radiates the input power effectively. These two bandwidths, as is known to those skilled in the art, most substantially be the same for a well designed circularly polarized array. Larger axial ratio bandwidth is achieved at the expense of implementing dual feed to the elements resulting in more feed line loss of signal and consequent reduction in efficiency. To provide adequate scanning capability and higher gain for a given array, the radiating elements in an array must be arranged with smaller spacing but sufficient to incorporate the feed structure with tolerable minimum feed structure coupling. A good array antenna design must take into account the actual communication system requirement and provide an optimum balance between conflicting design requirements.
The fundamental concept of generating circularly polarized electromagnetic fields by means of simultaneous sequential rotation and phasing (SSRP) of N independent linearly polarized fields is the revolutionary invention of Nikola Tesla (U.S. Pat. No. 381,968, May 1, 1888) that placed him in the U.S. National Inventor's Hall of Fame. This technique, for N=2 applied to a single square or circular microstrip element capable of supporting two orthogonal degenerate (same resonant frequency) linearly polarized modes, has been used as described before, to produce circularly polarized microstrip antennas as shown in U.S. Pat. No. 3,921,179.
In U.S. Pat. No. 4,866,451 (Chen) there is disclosed a circular polarization technique for a microstrip array antenna which utilizes dual feed to the radiator elements. This description is solely concerned with the improvement of axial ratio bandwidth and does not at all address the important practical issue of antenna efficiency. The four element subarray in the design disclosed therein requires seven hybrid power dividers, each requiring a lumped resistance termination. The fact is that if quadrature hybrid power dividers are to be used for exciting each individual element in the subarray, the axial ratio bandwidth will be very good enough that further improvement by sequential rotation and phasing of the 2.times.2 array may not be necessary. A further drawback is that each element requires two orthogonal feed with vertical coaxial feed pins from the bottom which is inconvenient to fabricate and is often electrically unreliable for pure circular polarizations at frequencies above 15 GHz. A more serious drawback is that accommodation of these seven hybrids within the array unit cell requires larger area and space, thus severely limiting the electronic scanning capability of the array.
While arrays of individual microstrip radiators are primarily used to increase the antenna gain, if-electronic scanning is an additional requirement for the array then there is necessity of placing restrictions on the element spacings to prevent the appearance of grating lobes during scanning. The four element cluster, acting as a building block for a larger array, then, is provided with phase shifters to provide electronic scanning. The entire coplanar feed structure must be accommodated within the confines of the four element cluster in such a fashion that detrimental inter-feed line coupling is minimized.
In order to improve upon the axial ratio bandwidth of a circularly polarized array of single feed structurally perturbed elements, Teshirogi in U.S. Pat. No. 4,543,579 has applied this well known SSRP technique of Tesla to a subarray of such elements implemented by a coplanar microstripline feed structure. There is an appreciable improvement on the available axial ratio bandwidth but that may not be sufficient for many wideband communication applications. Further, since sequential rotation and phasing is applied in two stages to the multi-element array, such antenna was not designed and is ill-suited for electronic scanning capability.
Applying the SSRP technique of generating a circular polarization signal, a two element subarray building block has been constructed and described by Haneishi and Suzuki (J. R. James and P. S. Hall Editors, Handbook of Microstrip Antennas Handbook, 1989, Peter Peregrinus Ltd. (IEE), London, Chapter 4, pp. 270-272) and Ito, Teshirogi and Nishimura (Chapter 13, pp. 804 of ref. as above). This two element unit employs structurally perturbed circular polarizable elements with single coplanar microstrip line feed provided by T-junction power dividers and extra 90.degree. phase delays provided by additional path lengths. Circular polarized microstrip elements with dual feed provided by coplanar microstripline T-junction power dividers are well known in the literature (J. R. James and P. S. Hall Editors, Handbook of Microstrip Antennas, 1989, Peter Peregrinus Ltd. (IEE), London, Chapter 4, pp. 221). Using such elements, Sreenivas in U.S. Pat. No. 5,231,406 has constructed a modified two element building block with a staggered arrangement that leads to a triangular grid array. Axial ratio bandwidth improvement has been considered, in isolation, as the design goal without concurrent attention to the antenna gain, antenna size and efficiency. The axial ratio bandwidth improvement has been proposed at the expense of undesirable loss of antenna gain. This is evidenced by the fact that there are only eight elements in the array area of 16d.sup.2 where d is the distance between two consecutive rows or columns in the array and the feed structure layout does not uniformly utilize the available space. This results in a nearly 50% loss in array antenna gain for a given array area caused by the loss in the antenna effective area.
For communications at higher microwave frequencies there is a present need for an optimally configured denser packed circularly polarized microstrip array that will eliminate the necessity of using quadrature hybrids without sacrificing the axial ratio performance obtainable from dual feed elements. It should be of simple construction and permit electronic scanning. It should also be realizable in a single conducting thin film so that very high antenna efficiency could be obtained by drastic reduction of feed line losses with realization of the array antenna in high temperature superconducting thin films.
It is therefore an object of the present invention to provide an optimum circularly polarized microstrip array antenna design wherein the axial ratio bandwidth is equal to or better than the impedance bandwidth and also wherein the variation of axial ratio over the entire beamwidth and bandwidth of interest is minimized without undue sacrifice of antenna gain and efficiency. It is also an object to provide a robust microstrip array antenna with dual feed elements that will radiate highly pure circular polarization over the frequency band of interest, is realizable in a single conducting layer thin film, employs an efficient and compact topology, makes optimum use of the unit array area and space without sacrificing performance, and maintains an excellent capability of electronic scanning.